A Kinesin-Like Protein Is Essential for Oriented Deposition of Cellulose Microfibrils and Cell Wall Strength

Abstract

Cortical microtubules have long been hypothesized to regulate the oriented deposition of cellulose microfibrils. However, the molecular mechanisms of how microtubules direct the orientation of cellulose microfibril deposition are not known. We have used fibers in the inflorescence stems of Arabidopsis to study secondary wall deposition and cell wall strength and found a fragile fiber ( fra1 ) mutant with a dramatic reduction in the mechanical strength of fibers. The fra1 mutation did not cause any defects in cell wall composition, secondary wall thickening, or cortical microtubule organization in fiber cells. An apparent alteration was found in the orientation of cellulose microfibrils in fra1 fiber walls, indicating that the reduced mechanical strength of fra1 fibers probably was attributable to altered cellulose microfibril deposition. The FRA1 gene was cloned and found to encode a kinesin-like protein with an N-terminal microtubule binding motor domain. The FRA1 protein was shown to be concentrated around the periphery of the cytoplasm but absent in the nucleus. Based on these findings, we propose that the FRA1 kinesin-like protein is involved in the microtubule control of cellulose microfibril order.

The rise and fall of the phragmoplast microtubule array

“…In vertebrates, the overlap length of PRC1-bundled anti-parallel MTs is controlled via overlap length-dependent microtubule growth inhibition by the Kinesin-4 motor, after it reaches the plus ends via its processive motility that is enhanced by a direct interaction with PRC1 [29 ]. In plants like A. thaliana, Kinesin-4 motors have been assigned to regulate cell elongation in interphase [30]. Instead, the Kinesin-12 motors have been implicated in regulating phragmoplast integrity [31].…”

Cytoskeletal Motors in Arabidopsis. Sixty-One Kinesins and Seventeen Myosins

“…It will be very interesting to explore whether some of these Arabidopsis kinesins play similar roles as cytoplasmic dyneins in animal and fungal cells in, for example, nuclear migration. The AtFRA1 kinesin, which belongs to the Kinesin-4 or KIF4/chromokinesin subfamily, was identified by an elegant screen for fragile fiber mutants (Zhong et al, 2002). Animal kinesins in this subfamily typically play roles in chromatid motility and chromosome condensation, activities associated with mitosis (Wang and Adler, 1995;Kwon et al, 2004;Mazumdar et al, 2004).…”

Mitosis-specific kinesins in Arabidopsis

“…The AtFRA1 homolog is not up-regulated during cell division and does not appear to be implicated in cell division. Indeed, its role is more consistent with the orientation of cellulose microfibrils in cell walls along with the MTs in the cortical array [61]. These findings underline the risk of drawing conclusions about the function of kinesins based on the extrapolation of amino acid conservation of the motor domain.…”